Various kinds of liquid crystal devices having a memory effect have been proposed.
U.S. Pat. No. 3,578,844 has disclosed a liquid crystal device in which a cholesteric liquid crystal material capsulated by a polymer compound such as gelatin or gum arabi is held between a pair of bases. According to the disclosure, this liquid crystal device has a memory effect, and attains a predetermined display state when a voltage is applied thereto, and this display state will be stably maintained even after stopping the application of a voltage. This is liquid crystal device performs the display based on a difference in quantity of reflected light, which is caused by applying the voltage to change the orientation state of cholesteric liquid crystal material having a selective reflection wavelength in a visible range.
The above liquid crystal device having the composite layer or film, which includes the polymer material and the cholesteric liquid crystal material, does not require a polarizer because it utilizes selective reflection of incident light by the liquid crystal material. Therefore, it is capable of bright display of the reflection type. Further, high-resolution display can be performed by simple matrix driving without using a memory element such as a TFT or an MIM.
The liquid crystal device of the reflection type utilizing the selective reflection of the cholesteric liquid crystal material changes the display state by selectively attaining the planar orientation and the focal conic orientation. In the planar orientation, helical axes of the liquid crystal molecules forming each domain are perpendicular to the base. In the focal conic orientation, the helical axes of the liquid crystal molecules forming each domain are irregularly directed or substantially parallel to the base.
FIGS. 7(A) and 7(B) schematically show an example of a conventional liquid crystal device having a composite layer, which includes a polymer material and a liquid crystal material exhibiting a cholesteric-characteristic.
In this liquid crystal device, a composite layer 3 is retained between a pair of transparent bases or plates 1a and 1b opposed to each other. Transparent conductive films 2a and 2b are formed on inner surfaces of the bases 1a and 1b, respectively. The composite layer 3 is made of, e.g., a mesh structure 3b of resin and a liquid crystal material 3a filling a space in the resin structure 3b. A black light absorbing layer 4 is arranged on the outer side of the transparent base 1b. The liquid crystal material 3a exhibits a cholesteric characteristic, and displays a predetermined color by reflecting the light of the selective reflection wavelength corresponding to the helical pitch length when it is in a planar orientation shown in FIG. 7(A), if the selective reflection wavelength is in the visible range. In the focal conic orientation shown in FIG. 7(B), it displays the background color, i.e., black. If the helical pitch length is relatively long, e.g., in such a case that the selective reflection wavelength is in an infrared range, the liquid crystal material 3a reflects the light in the infrared range to exhibit a transparent appearance when it is in the planar orientation shown in FIG. 7(A), and exhibits an opaque appearance when it is in the focal conic orientation shown in FIG. 7(B). Accordingly, this liquid crystal device can perform the mono-color display between the selective reflection color (planar orientation) and the background color (focal conic orientation) or between the background color (planar orientation) and white (focal conic orientation). For driving this liquid crystal device, a predetermined pulse voltage is applied across the conductive films 2a and 2b from a power source (not shown) for switching the state of the liquid crystal material 3a between the planar orientation and the focal conic orientation.
For attaining a multi-color display by the liquid crystal device having the composite layer which includes the resin and the liquid crystal material exhibiting the cholesteric characteristic, the liquid crystal device may have a layered structure including multiple composite layers which are layered together and can attain the planar orientations exhibiting different colors, respectively. An example of the liquid crystal device of the multi-layer type is shown in FIG. 8. The structure of the liquid crystal device shown in FIG. 8 was devised by the inventors and others during development of the invention.
This liquid crystal device includes three composite layers 3A, 3B and 3C, each of which is held between a pair of transparent bases or plates. These layers 3A, 3B and 3C reflect visible rays of different wavelengths and thereby exhibit different colors, respectively, when they are in the planar orientation. The composite layer 3A is held between the transparent bases 1A and 1B which are provided with transparent conductive films 2A and 2B opposed to the layer 3A, respectively. The composite layer 3B is held between the transparent bases 1B and 1C which are provided with transparent conductive films 2C and 2D opposed to the layer 3B, respectively. The transparent base 1B is commonly used for holding the composite layers 3A and 3B, and the transparent conductive films 2B and 2C are arranged on the opposite surfaces thereof, respectively. The composite layer 3C is held between the transparent bases IC and 1D, which are provided with transparent conductive films 2E and 2F opposed to the layer 3C, respectively. The transparent base 1C is commonly used for holding the composite layers 3B and 3C, and carries the transparent conductive films 2D and 2E on its opposite surfaces, respectively. A black absorbing layer 4' is arranged on the outer side of the transparent base 1D.
The composite layers 3A, 3B and 3C are formed of resin structures 3bA, 3bB and 3bC of, e.g., mesh forms, and liquid crystal materials 3aA, 3aB and 3aC filling the spaces in the resin matrixes 3bA, 3bB and 3bC, respectively. Liquid crystal materials 3aA, 3aB and 3aC exhibit red, green and blue appearances when they are in the planar orientation, respectively, and exhibit transparent appearances when they are in the focal conic orientation. FIG. 8 shows the liquid crystal materials 3aA, 3aB and 3aC in the planar orientation.
The transparent conductive films 2A-2F form electrodes, each of which takes the form of a matrix, and forms pixels with respect to the corresponding composite layer.
When driving this liquid crystal device, the display states of the composite layers 3A, 3B and 3C are individually controlled by controlling application of voltages from the power source (not shown) across the transparent conductive films 2A and 2B, across transparent conductive films 2C and 2D, and across the transparent conductive films 2E and 2E, respectively. Thereby, the display in multiple colors and, more specifically, eight colors including black can be performed on a predetermined pixel.
However, the following disadvantages arise when performing the multi-color display by the liquid crystal device of the above layered type.
First, all the composite layers are set to the focal conic orientation to attain transparent states so as to perform black display. In this case, a large quantity of incident light is reflected by the base surfaces and thus the degree of transparency lowers because the bases arranged between the viewer side and the black light absorbing layer are large in number. Consequently, the contrast is liable to be low.
Secondly, relative positioning between the respective pixels is difficult when overlaying the transparent bases provided with the transparent conductive films on each other.
Thirdly, the device is heavy and difficult to handle due to increase in number of the bases.
The above disadvantages can be avoided and further the multi-color display can be performed, if the liquid crystal device has the single composite layer, which is held between the paired bases, as is done in the liquid crystal device shown in FIG. 7, and is formed of the pixels capable of display in red, green and blue.
A photo-tunable method has been proposed as a method of manufacturing such a liquid crystal device. In the photo-tunable method, a nematic liquid crystal material and a tunable chiral material (TCM) added thereto are used as a liquid crystal material exhibiting the cholesteric characteristic. This tunable chiral material exhibits various chiralities depending on the degree of exposure to ultraviolet rays. According to the photo-tunable method, an assembly of a pair of bases, at least one of which is transparent, and a spacer disposed therebetween is prepared. The bases are provided with electrodes opposed to each other. A space between the bases is filled with a solution of liquid crystal material exhibiting cholesteric characteristic and resin monomer in compatibility state. Then, the assembly is radiated with ultraviolet rays to cure the resin monomer. This radiation with ultraviolet rays is performed through a filter for controlling the degrees of exposure of respective portions. Thereby, the liquid crystal materials in the respective portions of the composite layer have different helical pitch length and thus different selective reflection wavelengths. Therefore, the liquid crystal device can display different colors on the respective portions when the liquid crystal material is in the planar orientation.
According to the photo-tunable method, however, the base pair holding the compatible solution, which contains the resin precursor and the liquid crystal material, is entirely radiated with the ultraviolet rays for controlling the helical pitch length of liquid crystal material in the respective portions and for polymerization and phase separation of the resin. Therefore, it is impossible to control the position where the resin structure is formed, and the liquid crystal region in the produced composite layer has a continuous form. Consequently, the liquid crystal materials having different selective reflection wavelengths flow and will be gradually mixed with each other so that the multi-color display will not be performed stably. Further, the selective reflection wavelength depends on the degree of exposure to the ultraviolet rays so that the selective reflection wavelength of the produced liquid crystal device may change over time.